1. Field Of The Invention
The present invention relates to a combined cycle electric power plant and more particularly to improved superheat temperature control for heat recovery steam generators, as particularly adapted for use in combined cycle electric power plants.
2. Description Of The Prior Art
In the design of modern electric power plants, it is a significant object to achieve the greatest efficiency possible in the generation of electricity. To this end, steam generators are designed to extract heat efficiently from and to use the extracted heat to convert a fluid such as water into superheated steam at a relatively high pressure. Further, such steam generators have been incorporated into combined cycle electric generating plants including both gas and steam turbines wherein the exhaust gases of the gas turbine are used to heat water into steam then to be transferred to the steam turbine. Typically, steam generators include a water heating section or economizer tube, a high pressure evaporator tube and finally a superheater tube, whereby water is gradually heated while increasing levels of pressure are applied thereto to provide from the superheater tube, superheated steam to supply the steam turbine. A condenser is associated with the steam turbine to receive the spent steam therefrom and for converting it into water condensate to be fed back to the steam generator.
In a combined cycle electric power plant, the steam turbine is combined with a gas turbine whereby the heated exhaust gases of the gas turbine, otherwise lost to the atmosphere, are used to heat the circulated fluid and to convert it into steam to drive the steam turbine. In this manner, a significant reduction in the fuel required to heat the steam is achieved and the heat contained in the gas turbine exhaust gases is effectively utilized. Further, an afterburner associated with the exhaust exit of the gas turbine serves to additionally heat the gas turbine exhaust gases, whereby the heat required to generate sufficient steam to meet load requirements is provided. In particular, under conditions of relatively high loads, wherein the heat of the gas turbine exhaust gases is insufficient to supply the steam requirements, the afterburner is turned on to further heat the gas turbine exhaust gases.
In steam heaters of the prior art, there has been suggested that control of the superheated steam may be maintained by bypassing a portion of the steam derived from a steam drum of the steam generator, through a separate conduit about the superheater tube, whereby the relatively cold bypassed steam and the relatively hot superheated steam are recombined in selected proportions to achieve the desired temperature. For example, in U.S. Pat. No. 1,779,706, such a steam generator is suggested whereby primary and secondary superheater tubes are provided with a bypass conduit disposed about the primary superheater. The temperature of the steam passing from the steam generator ouput is measured and applied to a controller whereby the flow through the bypass conduit is controlled. The noted U.S. Pat. No. 1,779,706 does not, however, disclose that such control may be incorporated into or adapted to solve the particular problems of a combined cycle electric power plant.
Further, there is known in the prior art to spray or otherwise inject condensate water as derived from the condenser associated with the steam turbine into the fluid path of the steam generator. For example, the condensate water as driven by the main feed pump may be sprayed into the steam generator at a point intermediate a primary superheater tube and a secondary superheater tube. Thus, a valve may be selectively opened and closed to introduce the condensate water into the steam generator, whereby the temperature of the superheated steam may be correspondingly varied. In particular, the temperature of the superheated steam as derived from the steam generator and supplied to the steam turbine is measured and this variable is used to control the position of the condensate water inlet valve.
In the operation of a steam generator or a heat recovery steam generator as incorporated in the combined cycle electric power plant, it is particularly desirable to control within a minimum range the temperature of the superheated steam as supplied to the steam turbine, whereby the power generating efficiency of the electrical power plant is maintained at a relatively high level. In the prior art systems where it is attempted to achieve superheated steam temperature control by solely bypassing a portion of the fluid about a heat exchange tube of the steam generator or to inject feedwater at an intermediate portion thereof, it has been found that such temperature control is relatively unstable and that wide ranges of superheated steam temperature result for a given load demand. For example, for a steady-state load demand, prior art superheated steam temperature controls are typically capable of achieving superheated steam temperatures that vary over a range of approximately 1%, whereby a correspondig deviation in the megawatt output will result. Similarly, where the load demand signal is varying, the prior art superheater steam temperature controls are capable of maintaining regulation to only 2% with a corresponding drop in the power input. By contrast, the superheated steam temperature control of this invention achieves a superheated steam temperature control in the order of 0.5% and a corresponding control of the power output at both steady-state and varying load demands.
The description of prior art herein is made on good faith and no representation that any prior art considered is the best pertaining prior art nor that the interpretation placed on it is unrebuttable.